Magnetic recording media have a magnetizable layer which imparts characteristic properties such as coercivity (H.sub.c) and residual magnetic flux density (B.sub.r) to the media. Growing demands for high density recording media require such properties to be maximized.
Magnetic powders of acicular Fe metal, gamma-Fe.sub.2 O.sub.3 (mag-hematite) or Fe.sub.3 O.sub.4 (magnetite) are widely used in magnetizable coatings. These particles preferably exhibit high dispersibility in the coating slurry, high packing density in the coated film and high coercivity. Such particles typically have a narrow size distribution, a relatively large aspect ratio (major axis/minor axis) and are substantially free of dendrites (particle branching) or other irregularities.
Known acicular or spindle-shaped magnetic particles can be prepared by dehydrating acicular goethite (alpha-ferric oxyhydroxide) to form hematite (alpha-Fe.sub.2 O.sub.3), reducing to Fe metal or magnetite (Fe.sub.3 O.sub.4), and oxidizing at temperatures of at least 200.degree. C. to gamma-Fe.sub.2 O.sub.3. The gamma-Fe.sub.2 O.sub.3 particles may be further modified with cobalt by known methods to increase their coercivity.
The size, shape and distribution of the starting goethite particles directly affect the size, shape and distribution of the resulting magnetic particles. Therefore, goethite particles which have a spindle or acicular shape with sufficiently large aspect ratio, are uniform in size, and are substantially free of dendrites will contribute to obtaining the desired magnetic properties of the finished magnetic recording media.
Several processes are known for producing acicular goethite particles. In a first method, the goethite may be prepared at pH less than 6 by first reacting a ferrous salt and an alkali hydroxide in solution to precipitate a ferrous hydroxide precursor and then oxidizing the precipitate slurry to goethite. One of the major problems with this method is that sulfur remains as an impurity in the goethite, and this tends to promote sintering upon reduction to magnetite or iron metal. The particles also tend to be bundled.
Goethite may also be prepared by oxidizing a slurry of ferrous hydroxide in excess caustic so that the pH is greater than about 11. The high pH process produces goethite particles with high aspect ratio and low sulfur content, but the particles tend to have a broad size distribution.
Another method involves oxidizing goethite from ferrous carbonate, rather than ferrous hydroxide, with pH generally between 8 and 11. This process tends to produce uniform, spindle-shaped particles with low sulfur content and few dendrites, but with small aspect ratios, if the oxidation temperature is held between about 30.degree. C. and 50.degree. C. If the temperature is raised above 50.degree. C., the size uniformity of the goethite may deteriorate and cubic magnetite impurities tend to form.